Light produced in the Fibre bundle/Light guide/PMT region of the Tile Calorimeter prototype

Muons of 10-300 GeV crossing the Tile calorimeter loose some fraction of their energy ( 3 GeV in the whole calorimeter).The cells where the energy is deposited depend on the muons direction of incidence.Signicative signals ( 350 MeV) were observed in cells where the signal should be null.These extra-signals are of the same order of magnitude as the direct signal produced by m uons of 10-300 GeV in a cell.This signal, produced by ionization or Cherenkov light, is produced in the bre bundle-light guide-PMT region, mostly in the PMT photocathode.The preliminary results of some scans with a radioactive source on a Hamamatsu R5900-8 dynodes phototube show instead no signicant eects.